Coiled



'1, Patented Mu. 13, 194s COILED FILAMENT OR CATHODE AND` METHOD OFMANUFACTURE William P. Zabel, East Cleveland, Ohio, assignor to GeneralElectric Company, a corporation of New York Application october 3o,1943, serial No. 508,312

(ci. 17e-4o) 12 Claims.

'I'his invention relates to coil structures for electric lamps anddischarge devices, and especially to electric discharge electrodes,including cathodes suitable. for fluorescent lamps or other tubes of lowpressure positive column discharge type. The invention is hereinafterexplained with particular reference to'coils of refractory metal for useas fluorescent lamp cathodes, a1-

though it may also be found useful for laments of incandescent electriclamps.

It is desirable that a fluorescent lamp cathode should heat up andbecome fully emissive very quickly. On the other hand, it is alsodesirable that the active cathode surface should all be at potentialswithin a restricted range during operation, if not at substantially thesame potential. When the cathode is of the activated type (embodying anelectron-emissive material of low work 'function, as distinguished frombare metal), it

is desirable that the cathode should hold and retain a plentiful supplyof activating material or oxide, such as a mixture of alkaline earthmetal oxides including those of barium and strontium. In addition, it isdesirable that the metal structure of the cathode should presentprominences or points from which the discharge can take 01T readily.

In the effort to meet these requirements, some rather complicated andexpensive cathode constructions have been proposed, such, for example,las a coiled coil or even a triple coil; or a coiled coil with anoverwind of iiner wire, which may either hug the main wire or surroundit loosely. However, I have now devised a coil construction which meetsthe requirements without necessity for multiple coiling of anycomponent-though a coiled coil might, indeed, be embodied in'it, iffound desirable. Accordingly, my coil is much simpler, easier, and moreeconomical to make than the complex structures above referred to, andcan be made on ordinary, standard ceiling machines. The inventionaffords the further advantage that in suitable forms of construction itgives a cathode which is adapted either for cold starting, or for hotstarting with preheating by preliminary current iiow through thecathode.

unactivated, because of its relatively low work function.

In the forms of cathode here illustrated and described, there is a coilof main conductor wire associated with other wire that is coiledsubstantially coaxially and co-ordinately with the main wire, so as tointervene between its convolutions. Thus the wires co-operate to form atubular openwork basket or grille for holding electron-emissiveactivating materiaL The size of the main conductor wire may be chosenprimarily with reference to the current to be carried, and that of theother wire to assure rapid heating thereof by the discharge-though ofcourse both wires must be taken into account in determining theelectrical resistance of the cathode. If desired, the main coil may be adouble or other multiple coil of several parallel wound Wires, and theother coil may likewise be a multiple one; but in general the desiredstiness, resistance, and other characteristics can be obtained withoutthe complication of multiple coils.

When coiled in the same helical sense (i. e., both as right-handhelices, or vice versa), the main and associated wires may be Wound todifference pitches so as to give a criss-crossing of these wires. But ingeneral, opposite coiling of these wires is preferable. Such a, grillageholds the activating material very effectively and securely; it oiiersmany crevices with strong capillary attraction for retaining thismaterial against repeated or continual positive ion bombardment; and theinterlocking wires coact and hold together in a thoroughly unifiedstructure. If desired, one

or more additional windings of ne wire might besuccessively wound onover the rst ne wire winding, thus increasing the closeness of thebasket or grid texture.

Various other features and advantages of the invention will appear fromthe description of species and forms of embodiment and from the drawing.

'Ille coils may be made of any metals suitable for Y the conditions ofuse, including even metals or alloys that might not ordinarily beconsidered refractory-such as nichrome, for example. Amongst refractorymetals, tungsten and molybdenum, which I term tungsten metals, may bepreferred for cathode coils that are activated with alkaline earthoxides, while tantalum` may be In the drawing,

Figs. 1 and 2 are somewhat diagrammatic fragmentary side viewsillustrating the winding of the several associated wires on a mandrel toform -a. cathode coil suitable for the purposes of the invention, andFig. 3 is a longitudinal sectional view of the resulting coil, afterheat treatment and removal of the mandrel.

Fis. 4 is a side view of the coil after mounting as part of afluorescent lamp cathode mount, but before impregnation with activatingmaterial; and Fig. 5 is a fragmentary side view of the impreferred forcathode coils that are used bare and pregnated coil on a larger scale.

Fig. 6 is a view similar to Fig. 2 illustrating a somewhat diii'erentwinding of the component wires of a cathode coil; and Figs. 7 to 10 areslmilar views illustrating still different windings.

All the figures are drawn much larger than the actual coils as theywould be made for the cathodes of ordinary fluorescent lamps.

Figs. l and 2 illustrate the fabrication of a form of my cathode coilaccording to a preferred method which I have found convenient andeconomical as compared with prior methods involving multiple coiling. Asshown in Fig. 1, a main conductor wire is iirst wound into a helix I Ion a mandrel wire M of suitable metal and size, with its adjacentconvolutions suitably separated. Thereafter, another wire of smallergauge is wound under tension on the same mandrel M into a helix I2 ofopposite pitch: i. e., if the helix I I is a left-hand one, the helix I2is a right-hand one, or vice versa. Accordingly, convolutions of wire I2-lie against the mandrel M between those of wire II and cross the latterin bends outside of and conformable to Wire II. The windings may beapplied to long lengths of mandrel wire by standard ceiling machinesused in the manufacture of coiled filaments for incandescent lamps. Asshown, the pitch of the right-hand helix I2 is considerably less thanthat of the left-hand helix II, being onlyabout one-third as great.Accordingly, there are three times as many turns of the wire I2 as ofthe wire II. As shown in Fig. 2, a couple of turns or convolutions ofthe helix I2 are seen to lie between adjacent turns or convolutions ofthe helix II in front of the mandrel M, while another turn I2 is seencrossing each turn Il. Thus the helix I2 is distorted by an outwarddisplace ment of the flner wire at each crossing I that it makes overthe helix II, and is, as it were, interlocked with the helix Il againstrelative displacement. Of course the crossings of wire II by wire I2behind the mandrel M do not appear in Fig. 2. But as the windings II, I2are oppositely wound and continuous, each convolution I2 must cross aconvolution II once, and each convolution I I is crossed by threeconvolutions I2.

After winding, the wires II, I2 undergo a heat treatment on the mandrelM to set them. While the prolonged heat treatment known in the tungstenmetal wire art as sintering" may be used in treating tungsten wire coilsII, I2, I have found it generally suilcient and preferable to use theless severe heat treatment known in the art as annealing," since thisremoves most of the stresses due to wire drawing and soiling, yet leavesthe wires II, I2 ductile, and so reduces loss in handling and shippingcoils and lamps. For annealing, the mandrel wire M with its coils II, I2may be run through an annealing furnace containing an atmosphere ofhydrogen, thus cleaning and annealing it at one operation. No specialprecautions as to drying the hydrogen are ordinarily required. When themandrel M is of molybdenum, a temperature of the order of about 1550 C.may be preferred, with an exposure of the windings thereto for a periodof the order of seconds, or not over half a minute. When the mandrel Mis of iron or of mild, low-carbon steel, the temperature may be of theorder of 1300 C., and should not substantially exceed this figure, inorder-to obviate alloying of iron with the tungsten of the windings II,I2 which would embrittle the tungsten, while the exposure may be of theorder of half that above indicated for a molybdenum mandrel. Even at1300 C..

too long an exposure of the tungsten windings II, I2 in contact with aniron mandrel M may similarly embrittle the tungsten. After the heattreatment, the assembly of coils on mandrel is cut to individual lengthsand then the mandrel M is removed from within the windings II, I2, whichmay readily be done by chemically disintegrating the mandrel, as bydissolving it out with any suitable reagent-such as the usualcombination of nitric and sulphuric acids for molybdenum, orhydrochloric acid for iron or mild steel'.

In winding the wires II, I2 on the mandrel M, they are highly stressedelastically, so that they tend very strongly to uncoil. This elasticstress is largely relieved in the heat treatment, bul, not entirely so;hence when the wires II, I2 are released and the mandrel M is dissolvedout, the wires readjust themselves and are irregularly displaced.

The annealed coil assembly (of which only a fragment appears in Figs.1-3) is cut into suitable lengths L which may be mounted in leadwireclamps 2li, 20 of a fluorescent cathode mount' 2i, Fig. 4, according tothe usual practice in mounting cathodes or filaments. As shown, theleads 20, 20 have the usual auxiliary anode extensions 22, 22 beyond theclamps. After mounting a coil L in lead clamps 20, 20, the latter may bebent further apart somewhat, so as to stretch the coi-1 L taut betweenthem. This also has the effect of definitely separating the adjacentconvolutions of the associated coils II, I2 to a desired spacing. Thiscompletes the unactivated cathode mount as shown `in Fig. 4.

The impregnation of the coil with activating material may be carried outwith the usual mixture of powdered alkaline earth metal carbonates inbinder of nitrocellulose lacquer and diluent, which may be applied witha spoon" in the usual way to cover the coil and ll its interior and itsinterstices. To take full advantage of the large space afforded by and-Within the tubular basket or grillage formed by the coils II, I2, theliquid activating mix may be thicker than usual for coiled coilcathodes, about of the consistency of very thick cream; but care must betaken to avoid leaving an accumulation of this mix hanging to theexterior of the coil between the leads 20, 20 or on the lead wireclamps. The resulting cathode mount is subsequently sealed into afluorescent lamp tube, and is treated and activated as usual in theprocessing of the lamp to break the carbonates down to oxides.

In the impregnated cathode as shown in Fig. 5, none of the convolutionsof either coil II or I2 are totally embedded in the activation material,which substantially fills the grillage formed by these coils, but onlycoats the outer sides of the wires very lightly, as indicated by thestippling. After activation of the cathode shown in Fig. 5, that is,breaking down ofthe carbonates to oxides, the outer sides of theconvolutions of coll II and the cross-over portions I5 of coil I2 have ametallic luster. Thus the finer wire I2 heats up quickly and facilitates.starting of the discharge,

fluorescent lamp operable on a voltage of 108 with.

a current of 0.41 ampere are here given, but are not to be understood asdefining or limiting the invention:

For the mandrel M may be used round molybdenum or iron wire of 23 milsize, while for the wires Il, I2 may be used round tungsten wires of 2.3and 0.7 mil sizes, both preferably what is known in the art as non-sagwire. The primary, main conductor wire II of 2.3 mil size may be woundleft-hand on the mandrel M at 172 turns per inch, while the secondaryassociated wire l2 of 0.7 mil size may be wound right-hand at 530 turnsper inch. Using a length L of the resulting coil about mm. long afterheat treatment and removal of the mandrel M, it may be clamped in leadclamps 20, with a length of 13 mm. of coil in circuit betweenthemcomprising lengths of 174 mm. and 533 mm. respectively of the wiresII and I 2-and the clamps 20, 20 may then be bent apart to stretch this13 mm. length to a length of 15 mm. in circuit between the clamps. Theoperations not here mentioned or described in detail may be performed asalready described.

Fig. 6 illustrates a modification in which wires li, I2 are wound on amandrel M to equal but opposite pitchesone left-hand and the otherright-hand, or vice versa. These pitches may be intermediate betweenthose of the wire II and I2 in Figs. 1 and 2, so that the intervalbetween adjacent turns II, I2 shall be about the same, while theover-all length of the resulting cathode coil is correspondinglyshorter. For a watt 108 volt fluorescent lamp operating on 0.41 ampere,the mandrel wire M may be of 35 mil size, the wire I I may be of 2.3 milsize and wound in a left-hand helix at 265 turns per inch, while thewire I2 may be of 1 mil size and wound righthand at 265 turns per inch.The length of coil between the lead clamps 20, 20 (Fig. 4),may be 10.7mm. and stretched to 11.5 mm.

Fig. 7 illustrates a modification in which the main heavier gaugeconductor wire II is wound left-hand, a finer wire. I2 is woundright-hand,Y

and another finer wire I3 is wound left-hand.

In Fig. 8, the finer wire I2 is wound at less turns per inch and in theopposite direction from heavier wire Il.

In Fig. 9, two finer wires I2a, 12b are coiled side-by-side around themandrel M and heavier wire coil II.

In Fig. 10, the ner wire I2 is wound in the same direction as theheavier wire II but at less turns per inch.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A coiled filament comprising main helically coiled conductor wire;associated finer wire helically coiled substantially coaxially andcoordinately with said conductor wire, with portions of its convolutionscrossing over and other portions intervening in the space betweenconvolutions of said conductor wire and interlocked therewith againstrelative displacement of the coils.

2. A llamentary cathode comprising main helically coiled conductor wire;associated finer wire helically coiled substantially coaxially andcoordinately with said conductor wire, and having portions of itsconvolutions crossing over and other portions intervening in the spacebetween convolutions of said conductor wire, whereby the coils areinterlocked against relative displacement; andelectron-emissive-activating material held in the openwork tubular grillcooperatively formed by the coordinate convolutions of said wires.

3. A coiled filament comprising main helically coiled conductor wire;associated finer wire helically coiled substantially coaxially andcoordi` nately with said conductor wire but to a different pitch, withportions of its convolutions intervening in the space between those ofsaid conductor wire and other portions of its convolutions crossing thelatter in bends outside of and conformable thereto.

4. A coiled filament comprising a single main conductor wire helix; asingle associated finer wire helically coiled to a coil of diierentdegree of and opposite pitch coaxial and coordinate with said conductorwire, and having portions of its convolutions intervennigin the spacebetween convolutions of the conductor wire and othenportions crossingthe latter in bends outside of and conformable thereto.

5. A. filamentary cathode comprising a single main conductor wire helix;a single associated finer wire helically coiled to a coil of the samepitch but opposite helical sense coaxial and coordinate with saidconductor wire, with its convolutions intervening between and crossingthose of the conductor wire in bends outside of and conformable thereto,and electron-emissive-activating material held in the openwork tubulargrill co-operatively formed by the coordinate convolutions of saidwires.

6. A coiled iilament comprising main helically coiled conductor wire;associated finer wire helically coiled substantially coaxially andcoordinately with said conductor wire in the same helical sense but to adifferent pitch, with convolutions intervening between those of saidconductor wire and also crossing the latter in bends outside of andconformable thereto.

7. A method for fabricating a complex iilamentary coil which compriseshelically winding elastic main conductor wire on a mandrel, with itsadjacent convolutions separated, and helically winding finer wire onsaid mandrel with convolutions of the ner wire lying between andcrossing over those of the conductor wire, and subsequently removing themandrel and permitting the convolutions of main conductor Wire to openup, whereby the coils are interlocked against relative displacement.

8. A method for fabricating a complex filamentary coil which compriseshelically winding elastic main conductor wire on a mandrel, with itsadjacent convolutions separated, and helically winding finer wire onsaid mandrel to an opposite pitch, with convolutions of the latter wirelying between and crossing over those of the conductor wire.

9. A method for fabricating a complex lamentary coil which compriseshelically winding elastic main conductor wire on Fa mandrel, with itsadjacent convolutions separated and elastlcally stressed, helicallywinding liner wire on said mandrel with convolutions of the latter wirelying between and crossing over those of the conductor wire, partiallyrelieving the elastic stress in said main conductor wire, and releasingsaid coiled wires and removing said mandrel from the coils.

10. A method oi' fabricating a complex lamentary coil which comprisescontinuously winding a main conductor wire on a long length of mandrelwith its adjacent convolutions separated and elastically stressed,continuously winding iiner wire under tension over said main conductorwire with convolutions of the liner wire crossing over those of theconductor wire and contacting the mandrel between convolutions of theconductor wire, cutting the resultant assembly to individual iilamentlengths, and then removing the mandrels from the individual iilaments.

l1. A method for fabricating a complex lamentary coil which compriseshelically winding elastic main conductor wire on a mandrel, with itsadjacent convolutions separated, helically winding finer wire on saidmandrel in the opposite helical sense and at more turns per inch thanthe main conductor Wire, with convolutions of the nner wire crossingover those ofthe main conductor wire and contacting the mandrel betweenconvolutions of the main conductor wire, and subsequently removing themandrel and permitting the convolutions of main conductor wire to openup, whereby the coils are interlocked against relative displacement.

12. A fllamentary cathode comprising main helically coiled conductorwire; associated liner wire helically coiled substantially coaxially andcoordinately with said conductor wire but in the opposite helical senseand at several-fold the number of turns per inch, with portions of itsconvolutions crossing over and other portions intervening in the spacebetween and within the convolutions of said conductor wire andinterlocked therewith against relative displacement of the coils; andelectron-emissive activating material held in the tubular grillcooperatively formed by the co-ordinate convolutions of said wires.

WILLIAM P. ZABEL.

